When teaching, a teacher usually writes using chalk and draws using an instrument on a blackboard. The instrument includes a ruler, a set square, and a protractor, and so on. The protractor is a semi-circular structure, where an edge of the protractor is labeled with scales for measurement, and to draw an arc, a circle, and a straight line. The set square is an isosceles right triangle, or a 30-60-90 degree triangle, which can help in drawing an isosceles right triangle, a 30 degrees right triangle, a 60 degrees right triangle, and a straight line. Each of educational rulers has different functions, therefore, the teacher usually needs to carry different rulers when teaching, and change the educational ruler when drawing different pictures, which makes it inconvenient to carry and use.